From cuprates and pnictides to other exotic classes of materials, we had always had fun making junctions and devices with various superconductors. Tunnel junctions and Josephson junctions can not only reveal basic properties of superconductors, but they can be used to explore technologically important devices.   

Our observation of perfect Andreev reflection as a manifestation of the Klein paradox in SmB₆/YB₆ topological superconducting bilayers has changed the way we fundamentally think about making junctions. We had observed that a tunnel barrier at topological super(S)/normal(N) interface disappeared due to Klein tunneling. Whenever you attempt to make an SNS Josephson junction with a new “exotic” superconductor, there is usually formation of an undesirable barrier at the S/N interface, which ends up dominating and ruining the overall properties of the junction in an uncontrollable way. For instance, we struggled with this undesirable interface resistance in YBa₂Cu₃O_7-x/Au/YBa₂Cu₃O_7-x junctions in the 80s and the 90s. Of course YBa₂Cu₃O_7-x is not topological, but many of us believe that this deleterious effect ultimately dashed the hope for electronics based on high T_c superconductors.

So it is good to know that as we continue to make more junctions incorporating topological insulators such as SmB₆, we have Klein tunneling on our side (if all the conditions are met) to help remove such undesirable interface resistance. We are now taking this a step further and moving forward with making more devices using SmB₆ and YB₆ thin films, where we are able to take advantage of true insulation SmB₆ layers exhibit at low temperatures. Of particular interests are trilayer Josephson junctions and their arrays.      

Since the publication of our work, we have been approached by several groups to see if we can try to observe Klein tunneling in other potential topological superconductors. The idea is to use Klein tunneling as the probe of helical topological states. This is intriguing since it is nontrivial to directly measure topological properties of materials. Seunghun Lee, the first author of the original paper, now an Assistant Professor at Pukyong National University in South Korea, is leading some of this effort. He is also working on exploration of new topological insulators and topological superconductors, which have been theoretically predicted, by growing them in thin film forms to verify their topological properties. 

To quote Victor Galitski, a senior author from the paper, "apart from these exciting applications, the topological superconducting Klein tunneling devices contain a variety of novel, fundamentally interesting physical phenomena involving topology and strong correlations. It is quite rare to see materials systems where cutting edge theoretical research and promise of practical technological applications intersect."

Ichiro Takeuchi